This invention generally relates to optical fiber sensors and more particularly to a configuration and construction of an optical fiber pressure sensor. The invention provides a sensor which may be applied to detecting pressure in a fluid medium such as saltwater.
Marked advancements have been made in optical fiber technology over the past dozen years. Improvements in optical fibers per se have led to a proliferation of sensors which are applied to the detection of many types of physical changes. For example, various configurations of sensors using optical fibers have been applied to detecting changes in temperature, pressure, sound, relative rotation, current, voltage, magnetics and other physical phenomena.
Generally, optical fibers exhibit a natural immunity to environmental influences including electromagnetic influences and this makes them valuable in the communications field wherein information is transmitted over long distances. However, such natural immunity poses a problem in the application of optical fibers to sensing changes in the physical environment and therefore special attention is given to manufacturing specific optical fiber constructions which may be packaged to respond sensitively to the environment.
Trends in optical fiber sensor technology are disclosed in the IEEE JOURNAL OF QUANTUM ELECTRONICS, Vol. QE-4 of April 1982 and in HIGH TECHNOLOGY, Vol. 2, No. 4 of July-August 1982 in an article by Jeff Hecht entitled "Fiber-Optics Turns to Sensing." In the latter reference, an acoustic sensor is described and illustrated in the configuration of a Mach-Zehnder interferometer. This type sensor is well documented in the literature and utilizes a single-mode optical fiber to detect the relative phase shift of light propogating in two arms which comprise the interferometer.
A particular configuration of a Mach-Zehnder pressure-sensing interferometer is taught in U.S. Pat. No. 4,162,397 to Bucaro et al, issued July 24, 1979. According to this patent, an optical fiber coil forms a sensing path of the interferometer and it is placed in a fluid medium while a second optical fiber which forms a reference path is isolated from any pressure change which may be present in the fluid. Thus, a pressure incident on the fiber coil changes the index of refraction of the coiled fiber to modulate the phase of light propogating within the coil. The light which propogates through the reference arm remains unmodulated and the two light rays are combined at the interferometer output to form an interference pattern which is directly related to the presence of a pressure change in the fluid.
An alternative to the Mach-Zehnder configuration fiber-optic interferometer is a polarimetric sensor which utilizes a single-mode optical fiber for the sensing function. The single-mode optical fiber supports two polarization eigenmodes exhibiting different phase delays and an interference pattern results from a mixing of the two polarization eigenmodes. Optical phase delay fluctuations within the fiber are converted to intensity fluctuations by optically mixing the two polarization modes at the fiber output. The theory of polarimeters is outlined by Marc D. Mermelstein, "High-Birefringence Fiber-Optic Polarimeters with Submicroradian Phase Delay Detectability", JOURNAL OF LIGHTWAVE TECHNOLOGY, Vol. LT-4, No. 4 (April 1986).
Fiber-optic sensors are very sensitive to temperature changes and therefore are excellent temperature sensors. A particular challenge is to develop a fiber-optic pressure sensor that is sensitive to pressure but which also exhibits a marked reduction in temperature sensitivity. A polarimetric sensor stabilized against ambient temperature changes is described in "Polarimetric Strain and Pressure Sensors Using Temperature-Independent Polarization Maintaining Optical Fiber" by Y. Kikuchi et al, 2nd International Conference on Optical Fiber Sensors, September 1984. According to this reference, temperature dependence of intrinsic fiber birefringence may be cancelled by the thermal stress of multi-layer plastic coatings.
The present invention is directed to a uniquely configured polarimetric sensor for detecting pressure and to a particular packaging and arrangement of the various elements which comprise the sensor. Because the invention is designed for application in a temperature-changing environment, special attention has been given to packaging such that the sensor sensitivity to temperature is minimized while the sensitivity to pressure is enhanced.